Many plastic packages, such as those made from poly(ethylene terephthalate) (PET) and used in beverage containers, are formed by reheat blow-molding, or other operations that require heat softening of the polymer.
In reheat blow-molding, bottle preforms, which are test-tube shaped injection moldings, are heated above the glass transition temperature of the polymer, and then positioned in a bottle mold to receive pressurized air through their open end. This technology is well known in the art, as shown, for example in U.S. Pat. No. 3,733,309, incorporated herein by reference. In a typical blow-molding operation, radiation energy from quartz infrared heaters is generally used to reheat the preforms.
In the preparation of packaging containers using operations that require heat softening of the polymer, the reheat time, or the time required for the preform to reach the proper temperature for stretch blow molding (also called the heat-up time), affects both the productivity and the energy required. As processing equipment has improved, it has become possible to produce more units per unit time. Thus it is desirable to provide polyester compositions which provide improved reheat properties, by reheating faster (increased reheat rate), or with less reheat energy (increased reheat efficiency), or both, compared to conventional polyester compositions.
The aforementioned reheat properties vary with the absorption characteristics of the polymer itself. Heat lamps used for reheating polymer preforms are typically infrared heaters, such as quartz infrared lamps, having a broad light emission spectrum, with wavelengths ranging from about 500 nm to greater than 1,500 nm. However, polyesters, especially PET, absorb electromagnetic radiation poorly in the region from 500 nm to 1,500 nm. Thus, in order to maximize energy absorption from the lamps and increase the preform's reheat rate, materials that will increase infrared energy absorption are sometimes added to PET. Unfortunately, these materials tend to have a negative effect on the visual appearance of PET containers, for example increasing the haze level and/or causing the article to have a dark appearance. Further, since compounds with absorbance in the visible light wavelength range (400 nm to 780 nm) appear colored to the human eye, materials that absorb and/or scatter visible light will impart color to the polymer.
A variety of black and gray body absorbing compounds have been used as reheat agents to improve the reheat characteristics of polyester preforms under reheat lamps. These conventional reheat additives include carbon black, graphite, antimony metal, black iron oxide, red iron oxide, inert iron compounds, spinel pigments, and infrared-absorbing dyes. The amount of absorbing compound that can be added to a polymer is limited by its impact on the visual properties of the polymer, such as brightness, which may be expressed as an L* value, and color, which is measured and expressed as an a* value, a b* value, and haze, as further described below.
To retain an acceptable level of brightness and color in the preform and resulting blown articles, the quantity of reheat additive may be decreased, which in turn decreases reheat rates. Thus, the type and amount of reheat additive added to a polyester resin may be adjusted to strike the desired balance between increasing the reheat rate and retaining acceptable brightness and color levels. It is clear that any means to limit the amount of reheat additive used would assist in improving the appearance of preforms and the bottles produced from them.
Due to aesthetic reasons, a blue tinge is normally desired in polyester beverage containers, especially containers for water applications. Polymer articles with a blue tinge tend to be more appealing to the human eye, and are thus generally preferred in these applications. Basic color theory indicates that yellow and blue are two complementary colors. It follows that the removal of one of these colors from the visible white light will lead to an article appearing to be the other color. For example, when yellow light is removed from the visible light, the article will appear to be blue-dominated.
Yellowness, which may be measured as b* values in the CIE color system, may thus be a particularly undesirable color in consumer packaging, and bluing agents such as cobalt and organic toners have been used to increase the blue tint of consumer packaging, thus shifting the b* value from yellow to blue (or from higher to lower b* values), creating a more appealing package. It would be ideal to simultaneously increase the reheat rate and decrease the rate at which color and brightness degrade, such as by increased yellowness, as the concentration of the reheat additive in a thermoplastic composition is increased. Because appearance is important in such packaging, and because bottles having a bluish tint have been very successful in the marketplace, and especially in the marketing of bottled water, it would be an additional advantage to provide a reheat additive that imparts a bluish tinge to the polymers in which it is used, thus acting as a bluing agent.
A further disadvantage of some conventional reheat additives known in the art is their instability during the PET manufacturing process. For example, antimony metal is known to re-oxidize to antimony oxide (which is ineffective at increasing reheat rate) if there are oxygen leaks in the melt-phase or solid-stating manufacturing processes. This results in variability in the heat-up rates of preforms in the reheat blow molding process and thus requires constant adjustments of the infrared lamp settings. It would clearly be an advantage to provide a reheat additive that may be relatively resistant to these re-oxidation effects.
While polyesters used for packaging, such as PET and its copolymers, have been adapted for use as containers for a wide range of consumer products, their inability to block ultraviolet (UV) light of certain wavelengths has made them less well-suited for use in the packaging of products subject to photo-degradation, such as fruit juices, soft drinks, wines, food products, cosmetics, shampoos, and products containing UV-sensitive dyes. Ultraviolet light is not visible to the naked eye, having a wavelength from about 100 nm to about 400 nm, and is subdivided into UV-C having a wavelength from about 100 nm to about 280 nm, UV-B having a wavelength from about 280 nm to about 315 nm, and UV-A having a wavelength from about 315 nm to about 400 nm. Although polyesters such as PET block much of the ultraviolet light from about 100 nm up to about 315 nm, they are less effective at blocking UV-A light from about 315 nm to about 400 nm. U.S. Pat. No. 4,617,374, related to the use of polymerizable UV-blocking agents (the disclosure of which is incorporated herein by reference in its entirety), describes some of the known effects of ultraviolet light on packaged products, and offers the ability to block a portion of the ultraviolet light to which the container is exposed by the use of such blocking agents. Clearly, an additive which may provide a polyester composition having improved reheat, or improved bluing, or improved UV-blocking, or any combination of these advantages, would make the resulting polyester article suitable in the packaging of a wide range of consumer products.
There remains a need in the art for polyester compositions containing additives that provide improved reheat without the problems associated with known reheat additives, such as re-oxidation and inconsistent reheat; improved brightness, clarity, and color; and that may also provide improved resistance of the contents to the effects of UV light.